Go implementation

Since I now ventured into learning [Go](http://golang.org/) I could not resist creating a Tennis Kata implementation.

I did the following modifications:
* Go has no ternary operator, so this was replaced by a corresponding if
* Single-lines in if-blocks need braces, so several "compact" statements are now bigger
* Methods in the TennisGame interface start with an uppercase letter, since only then they are public (exported)  -- Which, on the other hand, also easily recreates the smell of the public members in game 2.

Note that I could not test the installation part as described in the readme, though, "it should work" ;) After a merge I'll verify that.